200 Dr. G. J. Burch. [ Mar. 7, 
light without interfering with the free supply of air. By bringing the flame 
near the side of the chimney, and increasing or diminishing the supply of 
gas, the metal could be brought to a dull red heat only just visible. 
Operating at night in a room with the blinds drawn, the first appearance 
of luminosity was of a pearl-grey tint. Encouraged by this result, I proceeded 
to try how much fainter the light could be made in an absolutely dark room 
without becoming invisible. Accordingly, my father had the gas laid on in 
an inner room which had no windows. The bunsen burner, standing in the 
middle of the metal chimney, was lit and turned down low. After remaining 
in complete darkness for two hours I moved it to the side, so that the flame 
touched the metal, and waited. To my surprise, the first visible luminosity 
was dull red, instead of grey as in the previous experiment. — Slightly 
reducing the gas supply caused it to disappear gradually, but without passing 
through grey. \ 
It should be noted that I was disappointed at having obtained, after so 
much trouble, what seemed a worse result, and ascribed it to a too rapid 
raising of the temperature, but was unable by the most careful adjustment of 
the lamp to see any traces of the lght before it appeared red. The next 
night, after repeating the experiment with no greater success, I went to get a 
screw-clamp from the outer room, which was dark save for the light of a 
street lamp 100 yards away, upon the canvas sun-blind, which was drawn 
down. After returning, the first visible luminosity was grey until I 
had remained in darkness another half-hour, when I could see only red, 
as before. My original bias being opposed to the result obtained, gives 
greater weight to the experiment. 
In order to have another colour for comparison, I drilled a small hole in 
the back of the chimney on a level with the flame, the blueish light of which, 
received on a sheet of paper about 18 inches off, was just visible. This 
appeared of a rich blue tint, in strong contrast to the dark red of the metal. 
By varying the distance of the paper from the chimney the relative intensities 
of illumination could be adjusted until both reached the minimum simul- 
taneously. But the contrast of colour was always visible, however faint the 
light, when the eye was completely free from the after effects of previous 
stimulatoon. | 3 
Experiment 2.—Glowing metal did not seem satisfactory as a source of 
light, because the refrangibility of the rays increasing continuously with the 
temperature, there must be a quantity of green rays almost capable of 
exciting sensation by the time the red rays were strong enough to do so. I 
therefore modified the experiment, using the spectra of rarefied gases because 
